[wct] Child runners refactoring

[merlinnot]

I've spent a day trying to solve #229, but I find the code being too tightly coupled to make sure it works in all cases. There are two main reasons causing iframes not to be garbage collected:

• attached event listeners
• storing a reference to the same object in both outer and inner window

I've made a PR to fix the former, but the latter is much more complex due to data flying around both ways (shared state, error objects, statuses, ...). I feel like fixing the latter would require a deeper refactoring, preferably restricting all communication to postMessage and making it one-way only (like events).

What do you guys think? Maybe someone with a better understanding of a project would like to share some thoughts?

[merlinnot]

Closing due to inactivity.